Directional solidification is widely used in the Photo Voltaic [PV] industry to produce ingots, which are sliced into wafers and later processed to become solar cells. The current state of the art is dominated by a system where silicon is solidified directionally from the bottom to the top in a crucible of quartz.
The same principle can be used to refine silicon to produce feedstock for the PV industry. The directional solidification can then be used to control the impurities by segregation, both the absolute levels and relative levels of different elements (Al, Ca, Fe, Ti, Mn, B, P, etc.) can be adjusted. In addition, the process has to take into account the particles formed in the process and particles from the incoming silicon.
A disadvantage of today's approach is that the quartz crucible can be used only once, since it is destroyed due to a phase transition of the crucible material during cool down of the silicon ingot (and the crucible). In addition, an anti-sticking layer of, for example, Si3N4 is required for the quartz crucible approach, in order to avoid adhesion of the silicon.
It is an object of the invention to provide an improved solidification process which results in a reduced contamination of the ingot cast.
It is a further object of the invention to provide a system which can refine a molten material, such as silicon, without the need to replace the container for the molten material between the casting of each ingot.